1. Field of the Invention
The invention relates to an apparatus for measuring a spectral distribution of a printed product produced with a printing device, which comprises a illuminating means for illuminating the printed product, an optoelectronic measuring means for measuring the reflectance value of at least one section of the spectrum of the light reflected from the printed product, an optical dispersing means for dispersing the wavelengths of the reflected light, and a light entry gap plane that is definitive for the dispersing means.
2. Description of the Related Art
Various types of quality monitoring and control of printing processes are known in the graphics industry. One of the measuring methods used in this case is color measurement by means of an analysis of the spectral distribution of the light reflected from a printed product. Apparatus which measure spectrally resolve the spectrum of the reflected light in bandwidths of, for example, 20 or fewer nanometers and use these to determine the associated reflectance values. From the reflectance values, various characteristic color values can subsequently be derived. Breaking down the spectrum of the reflected light can be carried out by means of characteristic filters, prisms or diffraction gratings, gratings being preferred in modern apparatus because of their high resolution.
Various techniques are known to measure the reflected light broken down or diffracted by a diffraction grating in its various different spectral regions. In these techniques, amongst other things, the grating used is pivoted and in the process the light is directed onto a single photoelement. Alternatively, it is known to deflect a linear light beam, which is diffracted into a rectangular area, onto a diffraction grating through an entry gap. The light beam diffracted in this way is measured in this area by means of a line or surface sensor having a large number of photoelements. However, in all these apparatuses, complicated optics is needed between the product to be measured and the entry gap. Furthermore, optics, in particular prisms, is also needed between the grating and the measuring means.
The invention is based on the object of configuring an apparatus for measuring a spectral distribution of a printed product produced with a printing device of the type mentioned at the beginning more economically and more compactly with the same or improved measurement quality.